Volume and pressure dependences of the electronic, vibrational, and crystal structures ofCs2CoCl4: Identification of a pressure-induced piezochromic phase at high pressure

Abstract

This work investigates the high-pressure structure of $\mathrm{C}{\mathrm{s}}_{2}\mathrm{CoC}{\mathrm{l}}_{4}$ and how it affects the electronic and vibrational properties using optical absorption, Raman spectroscopy, x-ray diffraction, and x-ray absorption in the 0--15 GPa range. In particular, we focus on the electronic and local structures of $\mathrm{C}{\mathrm{o}}^{2+}$, since compression of $\mathrm{C}{\mathrm{s}}_{2}\mathrm{CoC}{\mathrm{l}}_{4}$ yields structural transformations associated with change of coordination around $\mathrm{C}{\mathrm{o}}^{2+}$, which are eventually responsible for the intense piezochromism at 7 GPa. This study provides a complete characterization of the electronic and vibrational structures of $\mathrm{C}{\mathrm{s}}_{2}\mathrm{CoC}{\mathrm{l}}_{4}$ in the Pnma phase as a function of the cell volume and the local $\mathrm{CoC}{\mathrm{l}}_{4}$ bond length, ${R}_{\text{Co-Cl}}$, as well as its corresponding equation of state. In addition, our interest is to elucidate whether the phase transition undergone by $\mathrm{C}{\mathrm{s}}_{2}\mathrm{CoC}{\mathrm{l}}_{4}$ at 7 GPa leads to a perovskite-layer-type structure where $\mathrm{C}{\mathrm{o}}^{2+}$ is sixfold coordinated, decomposes into $\mathrm{CsCl}+\mathrm{CsCoC}{\mathrm{l}}_{3}$, or it involves an unknown phase with different coordination sites for $\mathrm{C}{\mathrm{o}}^{2+}$. We show that $\mathrm{C}{\mathrm{o}}^{2+}$ is sixfold coordinated in the high-pressure phase. The analysis of optical spectra and x-ray diffraction data suggests the formation of an interconnected structure of exchange-coupled $\mathrm{C}{\mathrm{o}}^{2+}$ through edge-sharing octahedra at high pressure.